Use HHVM+EZC internal tidy
EZC doesn't currently support direct access to object properties via the
obj->std.properties hashtable, but tidy uses this extensively. But it
turns out that for production use cases, tidy_repair_string() should be
sufficient. $wgDebugTidy and $wgValidateAllHtml are not used, and
no deployed extension calls MWTidy::checkErrors().
The only difference I know of is that errors from tidy (status==2) lead
to the tidy output being used, rather than discarded. But
TY_(ReportFatal) has very few callers in tidylib -- probably none that
are reachable from stripped parser output.
So, throw an exception if MWTidy::checkErrors() is requested on an HHVM
instance with the tidy extension. For MWTidy::tidy(), use
tidy_repair_string(). Refactor some relevant code.